The present invention relates to a magnet roll used as a developing roll in electrophotography, electrostatic recording, etc.
In electrophotograph, electrostatic recording, etc., electrostatic image is formed on a surface of an image-bearing member (photo-sensitive body or dielectric body), and developed with a magnetic developer containing toner (one-component magnetic toner or two-component developer comprising toner and a magnetic carrier) conveyed to a developing region by a developing roll, and the resultant toner image is transferred to a transfer member (plain paper or the like) and fixed thereto is by heating and/or pressing.
Widely used as a developing roll is, for example, a magnet roll assembly having a structure as shown in FIG. 4. Referring to FIG. 4, a magnet roll 1 comprises a cylindrical permanent magnet 11 having a plurality of magnetic poles extending on its surface along a longitudinal direction and a shaft 12 fixed concentrically to a center portion of the cylindrical permanent magnet 11. As shown in FIG. 3, the magnet roll 1 is enclosed in a cylindrical sleeve 2 and supported by flanges 3a, 3b via bearings 4a, 4b at both ends of the shaft 12. The sleeve 2 and the flanges 3a, 3b fixed to both ends thereof are made of non-magnetic materials such as aluminum alloys, austenitic stainless steel, etc. A numeral 5 denotes a seal member (oil seal). With the above structure, a magnetic developer is attracted onto a surface of the sleeve 2 and conveyed to a developing region (region in which the image-bearing member is positioned in opposite to the sleeve) by a relative rotation of the magnet roll 1 and the sleeve 2 (for instance, by rotating the flange 3a while keeping the magnet roll 1 stationary) to develop the electrostatic image.
The cylindrical permanent magnet constituting the above magnet roll is usually an elongated one having an outer diameter D of 10-60 mm and a length L of 200-300 mm, L/Dxe2x89xa75, and formed of an isotropic sintered ferrite magnet, or an anisotropic bonded magnet mainly composed of ferromagnetic particles (Sr ferrite or Ba ferrite) and a resin (polyamides, chlorinated polyethylene, etc.). The anisotropic bonded magnet is produced, for instance, by heat-blending a mixture of starting materials, extrusion-molding or injection-molding the molten blend in a magnetic field and then magnetizing the molded product according to a magnetization pattern.
Toner and carrier are made finer to satisfy the recent demand of higher image quality, and the magnet roll tends to increase its magnetic force to compensate for a decrease in attraction thereof to the magnet roll. The magnetic force required for the magnet roll is about 500-800 G on a sleeve surface, suitable for almost all developing processes. However, there have been provided a developing process requiring as high a magnetic force as about 1000-1300 G.
The isotropic sintered ferrite magnets can be provided with any magnetic force distribution by integral magnetization, so that they are materials good in stability and extremely easy to use. However, they are disadvantageous in that their residual magnetic flux density Br is about 2000 G. resulting in a magnetic flux density limited to about 900-1000 G on a sleeve surface, failing to meet the demand of higher magnetic field.
On the other hand, the anisotropic bonded magnets can easily be provided with a residual magnetic flux density Br of about 2600 G, for instance, generating a magnetic field stronger than that of the isotropic sintered ferrite magnets. However, magnetic poles can be formed only in predetermined directions (anisotropic directions), limiting a magnetic force distribution. Further, orientation should be carried out in a magnetic field during molding, resulting in an anisotropic bonded magnet suffering from unevenness in magnetic properties in a longitudinal direction and poor productivity.
Moreover, the sintered ferrite magnets are hard ceramics, which are brittle materials poor in impact resistance and difficult to be worked, so that working relies on grinding in most cases. On the other hand, the bonded magnets can overcome such disadvantages of the sintered ferrite magnets. However, because they are anisotropic magnets oriented in a magnetic field, they are provided with magnetic poles only in fixed directions even in a cylindrical shape. As a result, they do not have the degree of freedom that any desired arrangement of magnetic poles are formed by magnetization, unlike the isotropic sintered ferrite magnets. In addition, because the anisotropic bonded magnets are long in size, uniform magnetic field orientation is difficult, and their variation in the longitudinal direction of the magnet roll also increases several times that of the isotropic sintered ferrite magnets, which raises the problem that image quality is greatly influenced in a magnetic brush development system sensitive to the uniformity of magnetic force.
Also, because the temperature coefficient of Br is as high as 0.2 %/xc2x0 C. in the sintered ferrite magnets, developing conditions may vary depending on the environment of use in high-image quality digital apparatuses having extremely high sensitivity in development, resulting in changes in developed images in some cases.
As described above, isotropic magnet materials having high magnetic properties have been desired as the solution to the problems (poor surface magnetic flux density, large variations in surface magnetic flux density, lack of the flexibility of magnetic pole formation, temperature changes in surface magnetic flux density, etc.) of the conventional magnet materials.
As magnets for meeting such a demand, isotropic bonded magnets comprising Ndxe2x80x94Fexe2x80x94B magnet powder, which have (BH)max of about 3 MGOe, have been proposed. However, the Ndxe2x80x94Fexe2x80x94B bonded magnets have a problem with regard to corrosion resistance, and are vulnerable to rusting, so that they should be coated with epoxy resins, fluororesins, etc. Long articles such as magnet rolls are likely to have coatings with defects, resulting in an increase in product cost. Also, in the case of magnet rolls, gaps between magnets and inner diameters of sleeves are generally set as small as about 0.5-1 mm to utilize surface magnetic flux densities of the magnets on the sleeves as efficiently as possible. Thus, if rust is generated on the magnets, the gaps between the magnets and the sleeves are clogged with rust, resulting in higher risk of locking accident.
Also, in long articles such as cylindrical permanent magnets for magnet rolls produced by extrusion molding, etc., magnet powder should be dispersed uniformly in the cylindrical permanent magnets to obtain uniform properties along the longitudinal direction, and magnet powder having a spherical shape is advantageous in uniform dispersion. Ferrite magnet powder is suitable for uniform dispersion because it has a particle size of about 1 xcexcm. However, Ndxe2x80x94Fexe2x80x94B magnet powder is in a shape of thin flake, so that it is difficult to be dispersed uniformly. If the Ndxe2x80x94Fexe2x80x94B magnet powder is pulverized to 100 xcexcm or less to achieve more uniform dispersion, it suffers from drastic deterioration of magnetic properties. Accordingly, although the bonded ferrite magnets are excellent in molding properties, they are low in surface magnetic flux densities and also poor in temperature stability. The Ndxe2x80x94Fexe2x80x94B bonded magnets are high in surface magnetic flux densities, but have a problem in respect to molding properties. They are also low in corrosion resistance and temperature stability. As described above, both magnets have many problems to be solved for applying them to the magnet rolls.
In view of such circumstances, a magnet roll comprising a cylindrical permanent magnet concurrently satisfying demands for surface magnetic flux density, temperature stability, corrosion resistance and molding properties have been desired.
It is therefore an object of the present invention to solve the problems of the conventional art, thereby providing an easy-to-use magnet roll having a high and uniform surface magnetic flux density.
The first magnet roll of the present invention has a plurality of magnetic poles on a surface, at least one magnetic pole portion comprising an anisotropic bonded magnet containing magnet powder and a binder resin, the anisotropic bonded magnet containing an Rxe2x80x94Txe2x80x94N magnet powder, wherein R is at least one rare earth element including Y, Sm being indispensable, T is Fe or Fe and Co, and O and H, inevitable impurities, may be contained, and the binder resin, a volume ratio of the binder resin being 20-70%. With this constitution, the Rxe2x80x94Txe2x80x94N magnet powder has substantially the same saturation magnetization as that of the Ndxe2x80x94Fexe2x80x94B magnet powder, so that the magnet roll has a high surface magnetic flux density. Further, the corrosion resistance can be improved drastically by reducing the C content of the Rxe2x80x94Txe2x80x94N magnet powder to a trace amount.
The bonded magnet of the present invention can have (BH)max of 10 MGOe or more, and Br of 2800 G or more with the amount of the binder resin adjusted.
The Rxe2x80x94Txe2x80x94N magnet powder has a temperature coefficient of a residual magnetic flux density Br of xe2x88x920.065%/xc2x0 C., much smaller than a temperature coefficient of Br of xe2x88x920.12%/xc2x0 C. of Ndxe2x80x94Fexe2x80x94B magnet powder and a temperature coefficient of Br of 0.2%/xc2x0 C. of a sintered Sr ferrite magnet. Accordingly, even when the magnet roll comprising a bonded magnet containing the Rxe2x80x94Txe2x80x94N magnet powder is used under severe conditions such as high temperature, continuous printing, etc. to elevate the temperature of the bonded magnet, the surface magnetic flux density of the magnet roll appearing on a sleeve surface varies only slightly, thereby stably providing high-quality image.
No rust is generated even when the above-described nitride magnet powder is exposed to a surface of the bonded magnet. Accordingly, even when the magnet roll is used in a high-temperature, high-humidity environment, there is no likelihood of locking, ensuring high reliability. Further, the average particle size of the Rxe2x80x94Txe2x80x94N magnet powder can be appropriately adjusted between 1 xcexcm and 10 xcexcm. Therefore, even when the ratio of the binder resin is changed for obtaining a required surface magnetic flux density, excellent moldability can be realized by changing the size of the Rxe2x80x94Txe2x80x94N magnet powder, thereby allowing the required surface magnetic flux density and excellent moldability to be compatible with each other.
The above-described Rxe2x80x94Txe2x80x94N magnet powder is preferably (a) a rare-earth magnet containing main ingredients having a composition represented by Rxcex1T100xe2x88x92(xcex1+xcex4)Nxcex4 by atomic %, wherein R is at least one rare earth element including Y, Sm being indispensable, T is Fe or Fe and Co, and xcex1 and xcex4 satisfy 5xe2x89xa6xcex1xe2x89xa618 and 4xe2x89xa6xcex4xe2x89xa630, respectively; O and H, inevitable impurities; and C in an amount of 5 atomic % or less based on the magnet powder, and having a magnetism-generating phase which is substantially a hard magnetic phase composed of a rhombohedral crystal having a Th2Zn17 structure and/or a hexagonal crystal having a Th2Ni7 structure, or (b) a rare-earth magnet containing main ingredients having a composition represented by Rxcex1T100xe2x88x92(xcex1+xcex2+xcex4)Mxcex2Nxcex4 by atomic %, wherein R is at least one rare earth element including Y, Sm being indispensable, T is Fe or Fe and Co, M is at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Cu, Ga, Zr, Nb, Mo, Hf, Ta and W, Ti being indispensable, and xcex1, xcex2 and xcex4 satisfy 5xe2x89xa6xcex1xe2x89xa618, 1xe2x89xa6xcex2xe2x89xa630 and 4xe2x89xa6xcex4xe2x89xa630, respectively; O and H, inevitable impurities; and C in an amount of 5 atomic % or less based on the magnet powder, and having a magnetism-generating phase which is substantially a hard magnetic phase composed of a rhombohedral crystal having a Th2Zn17 structure and/or a hexagonal crystal having a Th2Ni17 structure.
R should include Sm an indispensable element and may further include one or more of Y, La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Mixtures of two or more rare earth elements such as Sm misch metals or didymium may also be used. A combination of Sm and one or more of Y, Ce, Pr, Nd, Gd, Dy and Er is preferable as R, and a combination of Sm and one or more of Y, Ce, Pr and Nd is more preferable. Particularly preferable is Sm substantially alone. With respect to the content of Sm, the percentage of Sm in R is preferably 50 atomic % or more, and more preferably 70 atomic % or more, to obtain a high surface magnetic flux density. R may contain inevitable impurities such as O, H, Al, Si, Na, Mg, Ca, etc., whose inclusion is unavoidable in production processes, as long as the properties are not deteriorated.
The content of R is preferably 5-18 atomic %. When R is less than 5 atomic % or more than 18 atomic %, xcex1Fe is generated in a large amount, and other phases than a hard magnetic phase are generated, resulting in a decrease in surface magnetic flux density. The more preferable content of R is 6-12 atomic %.
Though the addition of a suitable amount of the M element slightly reduces the surface magnetic flux density, the bonded magnet is provided with extremely improved heat resistance, thereby providing a high-performance magnet roll suitable for continuous printing at high temperatures. The content of the M element is preferably 1-30 atomic %. When the M element exceeds 30 atomic %, an Sm(Fe, M)12Nz phase of a ThMn12 type is generated, resulting in decrease in surface magnetic flux density. On the other hand, when the M element is less than 1 atomic %, xcex1Fe is generated, likewise resulting in decrease in surface magnetic flux density.
The content (xcex3) of C is preferably 5 atomic % or less based on the magnet powder. More than 5 atomic % of C decreases a surface magnetic flux density and deteriorates corrosion resistance.
The content (xcex4) of nitrogen is preferably 4-30 atomic %. When the content of nitrogen is less than 4 atomic % or more than 30 atomic %, the surface magnetic flux density is low. The more preferable content of nitrogen is 10-20 atomic %.
In the above-described rare earth nitride magnet, 0.01-30 atomic % of Fe may be substituted by Co and/or Ni. The introduction of Co and/or Ni improves the temperature characteristics of the bonded magnet. However, when the amount of Co and/or Ni exceeds 30 atomic %, the surface magnetic flux density of the bonded magnet is significantly decreased. On the other hand, it is less than 0.01 atomic %, the addition effect thereof is not observed. The amount of Co and/or Ni substituted for Fe is more preferably within the range of 1-20 atomic %.
The pressure of a pure nitrogen gas or a nitrogen-containing gas in a nitriding treatment is preferably about 0.2-10 atm. The nitriding reaction is slow at lower than 0.2 atm, while the pressure of more than 10 atm necessitates a more expensive high-pressure gas apparatus. The more preferable pressure range of the nitriding gas is 1-10 atm. The heating conditions for the gas nitriding are preferably 300-650xc2x0 C.xc3x970.1-30 hours. Less than 300xc2x0 C.xc3x970.1 hour results in low nitrization, while more than 650xc2x0 C.xc3x9730 hours causes the generation of the Rxe2x80x94N phase and the Fexe2x80x94M phase, resulting in a significant decrease in surface magnetic flux density. The heating conditions for the gas nitriding is more preferably 400-550xc2x0 C.xc3x970.5-30 hours, and particularly preferably 400-550xc2x0 C.xc3x971-10 hours.
For realizing the uniform nitriding treatment and securing the easy molding of the bonded magnet, it is preferable that pulverization and classification are optionally carried out before nitriding to adjust a particle size of the magnet powder.
A magnet roll produced from a mixture of the above-described nitride-type rare earth (Rxe2x80x94Txe2x80x94N) magnet powder with ferrite magnet powder can not only be lower at cost than that produced from nitride-type rare earth magnet powder alone, but also higher in a surface magnetic flux density than a bonded magnet composed only of ferrite magnet powder.
The second magnet roll has a plurality of magnetic poles on a surface, at least one magnetic pole portion being composed of an anisotropic bonded magnet obtained by molding a mixture comprising nitride-type rare earth magnet powder of an Rxe2x80x94Txe2x80x94N alloy, wherein R is at least one rare earth element including Y, Sm being indispensable, T is Fe or Fe and Co, and O and H, inevitable impurities, may be contained, ferrite magnet powder and a binder resin in a magnetic field, a volume ratio of the binder resin being 20-70%.
As the ferrite magnet powder used in the present invention, ferrite powder having a main ingredient composition represented by A""O.nFe2O3 (atomic ratio), wherein A is Sr and/or Ba, and n (molar ratio)=5-6, is highly useful.
The ferrite magnet powder for the anisotropic bonded magnets suitable for the present invention can be produced, for example, by processes of mixing of starting material powderxe2x86x92ferritization by calcination (solid-phase reaction)xe2x86x92pulverizationxe2x86x92heat treatment xe2x86x92 disintegration (sieving) treatment. The average particle size of the ferrite magnet powder is preferably 0.8-2 xcexcm, more preferably 0.9-1.5 xcexcm. When the average particle size is outside this range, it is difficult to prepare a useful magnet roll.
The isotropic ferrite magnet powder can be produced by processes of mixing of starting material powderxe2x86x92ferritization by calcination (solid-phase reaction)xe2x86x92pulverizationxe2x86x92heat treatmentxe2x86x92disintegration (sieving) treatment.